pseudo-random number generator with trigonometry

For my assignment I need to implement a pseudo-random number generator. It takes in a start value (x) and seed. The seed must be generated based on the current date (ticks or something similar).

private long Rnd (double x)
{
   //Method that returns a datetime.now Ticks long
   var seed=CreateSeed(); 

   // x is the input the user provides
   // +1)/3 is added for variance
   long randomNumber = Convert.ToInt64((Math.Cos(90 * x * seed)+1)/3);

   return randomNumber;
}

What should I improve and change. Thank you for your help and advice.